Conventionally, an ultrasonic sensor has been proposed that is constructed from a piezoelectric element mounted on a base board made of metal, resin, or the like. In such an ultrasonic senor, an ultrasonic transmitter emits an ultrasonic wave, the emitted ultrasonic wave is reflected from an object to be detected, and an ultrasonic receiver receives the reflected ultrasonic wave. Based on the received ultrasonic wave, the ultrasonic sensor detects a location of the object, a distance from the object, a two-dimensional shape of the object, or a three-dimensional shape of the object.
An ultrasonic sensor disclosed in JP-A-H11-178823 includes a sensor probe constructed with a piezoelectric element for emitting and receiving an ultrasonic wave and an acoustic matching layer for increasing propagation efficiency of the ultrasonic wave. The piezoelectric element and the acoustic matching layer are coated with a high hardness packing material. Thus, the sensor probe is protected from degradation due to usage environmental factor such as moisture. However, since the piezoelectric element and the acoustic matching layer are entirely coated with the packing material, vibration of the received ultrasonic wave may be damped (i.e., weakened) by the packing material so that a detection sensitivity of the ultrasonic sensor may be reduced. Further, a detection error of the ultrasonic sensor may be caused by increased noise due to a resonant vibration of the packing material. Furthermore, since the packing material has high hardness, the package material may not absorb impact force applied to the sensor probe, and the sensor probe may be broken by the impact force. In addition, a detection error of the ultrasonic sensor may be caused by stress applied to the piezoelectric element due to thermal expansion and contraction of the packing material.